In the formation of integrated circuits (IC), thin films containing metal and metalloid elements are often deposited upon the surface of a semiconductor substrate or wafer. These thin films provide conductive and ohmic contacts in the circuits and between the various devices of an IC. For example, a thin film of a desired metal might be applied to the exposed surface of a contact or via hole on a semiconductor substrate, with the film passing through the insulating layers on the substrate to provide plugs of conductive material for the purpose of making interconnections across the insulating layers.
In processing semiconductor substrates or wafers to form an IC, sputter etching is a technique that is often used to remove a layer of unwanted material or an excess quantity of a material from the wafer surface. The process of sputter etching is generally known and takes advantage of the momentum of gas ions accelerated in an electric field. During sputtering, a gas is ionized and the gas ions are accelerated and collide with the surface of the material to be sputtered. During the collision, part of an ion's momentum is transferred to the surface of the material. The ionized particles of the charged gas plasma bombard the surface of the wafer and, if sufficient momentum is transferred, atoms and/or molecules are removed or etched from the surface.
In sputter etching, a gas is introduced into a processing chamber. The processing chamber may be metal, quartz or a dielectric other than quartz, and preferably is vacuum sealed. The wafer to be etched is supported on an electrical base or electrode within the reaction chamber so that the wafer develops an electrical potential or bias. A working gas is introduced into the vacuum chamber opposite the surface of the biased wafer, and energy is capacitively or inductively coupled to the gas through the processing chamber wall, such as by using an induction coil which surrounds the processing chamber. The energy from the induced field ionizes the gas particles so that they acquire a net charge that is of the opposite polarity to the potential of the wafer support and the wafer. The ionized particles of the gas collectively form what is referred to as a gas plasma or plasma cloud. Since the ionized particles of the plasma and the wafer are of opposite polarities, the ionized particles in the plasma are attracted to the wafer's surface, bombarding the surface of the wafer and dislodging material particles from the wafer to consequently etch the wafer surface.
For deposition processes, the sputter etching commonly occurs at wafer voltages of about 1000 volts (1 kilovolt). However, this relatively high voltage is inappropriate for microelectronic devices which are more susceptible to surface damage at these wafer charging voltages. As a result, lower wafer voltages, below 500 volts, are more desirable. Plasma etching that is accomplished using these lower wafer voltages and with a plasma generated independent from the bias on the wafer is referred to as a soft plasma etch.
The etch process occurs in a reaction chamber within which a low gas pressure is maintained. The gas, usually argon, is introduced and ionized via electron collision in an oscillating electromagnetic field (EMF). The accelerating voltage is supplied either by a separate radio frequency (RF) power supply, or in many cases may be the same power supply that provides for the ionization. Constant pressure is maintained by controlling the rates at which the sputtering gas is introduced into and is removed from the chamber.
The etch process is a first step in a variety of process sequences. One example is the fabrication of suicides where a contact surface is cleaned using sputter etch and a metal such as titanium (Ti) may be deposited over a metal oxide semiconductor structure to react with exposed silicon (Si), such as source and drain regions, to form metal silicides. Following the formation of the silicide regions, a selective acid etch has been used to remove unreacted metals without attacking the silicide. This removal is accomplished by completing the process to deposit the metal in the substrate, removing the substrate from the reactor, allowing the substrate to cool to room temperature and then etching the substrate with hydrogen peroxide, hydrogen peroxide containing a very small amount of ammonium hydroxide, or a mixture of hydrogen peroxide and sulfuric acid. This etch process removes any excess metallic Ti on the substrate as well as any substoichiometric titanium silicide (TiSi.sub.x) formed on the silicon dioxide. This method of forming silicides is disclosed in U.S. patent application Ser. No. 08/489,040 entitled METHOD FOR FORMING SILICIDES, filed Jun. 9, 1995 (inventor Arena) and assigned to Tokyo Electron Limited which is herein incorporated by reference in its entirety.
During the plasma etch process, SiO.sub.2 is removed or etched from the surface of the material. The plasma further dissociates this SiO.sub.2. The by-products of the SiO.sub.2, which include silicon monoxide (SiO) and atomic oxygen (O), are liberated and released into the plasma. The effect of the increased concentration of oxygen in the plasma, however, is a reduction in the rate of a SiO.sub.2 sputter process, such as a reduction in the sputter etch rate. A reduced sputter etch rate decreases the time efficiency, decreases wafer throughput and hence increases the cost of the entire process. Thus, a method is needed whereby sputter etch of SiO.sub.2 and other dielectric materials may be accomplished without the undesirable concomitant decrease in the sputter rate or having to resort to an increase in the ion energy.